In a device process and an epitaxial silicon wafer production process, a protective film and an insulating film are formed on a silicon wafer. For example, in the device process, an oxide film and the like are formed as an interlayer insulating film on the surface side of the silicon wafer to be used as a device fabrication region, then wiring formation and the like are performed.
In addition, in the production process of an epitaxial wafer, when a silicon epitaxial layer with high resistivity is vapor-deposited on a main surface of a silicon single-crystal substrate with low resistivity, a phenomenon that dopant in the silicon single-crystal substrate is once released to the vapor phase from the back surface etc. of the silicon single-crystal substrate and then doped in the silicon epitaxial layer, so-called, auto-doping, is liable to occur. Therefore, before performing vapor deposition, a silicon oxide film as a protective film for preventing auto-doping is formed on the back side of the silicon single-crystal substrate, the back side being free of epitaxial layer formation.
In general, when an oxide film is formed as an insulating film or protective film on a silicon wafer, an atmospheric CVD method is used. In the atmospheric CVD method, a silicon wafer is placed face up on a tray, with the film formation face being up, thereafter the tray and the silicon wafer are heated while supplying a source gas on the silicon wafer, whereby specific elements according as the source gas are deposited on the silicon wafer to allow film formation. The reason why the atmospheric CVD method is widely used when forming an oxide film is that due time for the formation of an oxide film can be shortened since the film-forming rate is high, and also that an oxide film can be continuously formed on the silicon wafer by incorporating a conveyance device.
In the atmospheric CVD method described above, a mixed gas of monosilane (SiH4) and oxygen (O2) or a mixed gas of tetraethoxysilane (TEOS, chemical formula: Si(OC2H5)4) and ozone (O3) is used as the source gas.
When an oxide film is formed by the atmospheric CVD method, a tray on which a silicon wafer is placed is required not to deform due to heating when forming a film, and not to be a cause for generation of contamination on the silicon wafer. Therefore, one obtained by sintering SiC or another obtained by further coating its surface with SiC film are used as the tray. As a general tray shape, a tray in which a holding portion for the silicon wafer is flat is used.
FIG. 10 is a cross section showing the state where a silicon wafer is placed on a conventional tray in which a holding portion is flat. A tray 1 shown in FIG. 10 has a flat holding portion 1a on which a silicon wafer 5 is placed. When film formation is performed on a film formation surface 5a of the silicon wafer by an atmospheric CVD method using the tray shown in FIG. 10, the opposite surface 5c of the silicon wafer, free of film formation, that does not allow film forming is in contact with the holding portion 1a of the tray when placing the silicon wafer 5 on the tray 1, and thus flaws occur over the whole opposite surface 5c, free of film formation, of the silicon wafer. Such contact-incurred flaws with a depth of about 3 to 10 μm occur while the depth of flaw varies slightly depending on film forming conditions.
When the formed oxide film is used as an interlayer insulating film, if rapid thermal history is provided to the silicon wafer by thermal treatment such as Flash Lamp Anneal in the device process and the like, there is a risk that the contact-incurred flaws induce cracking of the silicon wafer initiated from such flaws, and the product yield may be deteriorated. In addition, when the oxide film is used as a protective film, if an epitaxial layer is formed on the surface of the silicon wafer where contact-incurred flaws occur, there is a problem to generate stacking faults and the like in the epitaxial layer, initiating from such flaws. To deal with such a problem of generating contact-incurred flaws on the silicon wafer during film formation by an atmospheric CVD method, Patent Literature 1 uses a tray that supports the silicon wafer in its periphery region.
FIG. 11 is a cross section showing the state where a silicon wafer is placed on a conventional tray that supports the silicon wafer in its periphery region. A tray 1 shown in FIG. 11 has a tapered holding portion 1a, wherein the holding portion 1a supports the silicon wafer in its periphery region 5b, and a silicon wafer 5 is placed on the tray 1.
When the tray that supports the silicon wafer in its periphery region is used, the silicon wafer is supported so as not to allow a film-formation-free surface 5c of the silicon wafer to come in contact with the tray 1, and thus occurrence of contact-incurred flaws can be substantially reduced.
However, according to the experiment of the present inventors, it has been found that, when the tray that supports the silicon wafer in its periphery region is used, in a case of heating during film formation by an atmospheric CVD method, the surface temperature near the periphery of the silicon wafer is raised by heat conduction from the tray 1 to the periphery 5b of the silicon wafer to thereby vary the temperature distribution in the film formation surface 5a of the silicon wafer, and thus there is a problem that the thickness of the formed oxide film is made non-uniformly. Specifically, since the increase in the surface temperature accelerates the growth rate of the oxide film, an oxide film formed on the film formation surface 5a of the silicon wafer is thin in the central part and thick near the periphery.
Therefore, in the device process using the formed oxide film as an interlayer insulating film, when a silicon wafer with non-uniform thickness of the oxide film is used, there is a problem that device characteristics formed on the oxide film are notably deteriorated.
In addition, in recent years, it is desired to provide an epitaxial silicon wafer having high flatness, and when the thickness of oxide film in a silicon wafer for use is non-uniform, there is a problem that flatness of the epitaxial wafer to be formed thereafter is deteriorated.
Furthermore, when there is a flaw on the surface of the silicon wafer where an epitaxial layer is formed as described above, imperfections may be generated in the formed epitaxial layer. Therefore, after forming an oxide film, the film-formation-free surface is polished, as being one-side polishing, before carrying out epitaxial growth treatment, to carry out an operation to remove the oxide film, flaws, and the like.
However, during the treatment of one-side polishing, since the film formation surface is supported and the film-formation-free surface is to be polished, if the thickness distribution of an oxide film is non-uniform, the wafer is supported while elastically deformed, and non-uniform thickness distribution of an oxide film is transferred to the film-formation-free surface after polishing, and thus flatness of the silicon wafer is deteriorated. The more the polishing amount, the larger the deterioration of flatness, and it also affects to flatness of the epitaxial wafer to be formed thereafter.